[EPIC](alerts/ui): Alerts UI features and refactoring for general use

[edda]

There are various features that we want to add to the alerts UI. Additionally there are some areas where the app needs to be generalized and/or made configurable so that it becomes more usable in diverse landscapes.

Features:

• [x] #198
• [x] #200
• [ ] Number previously expired silences of a altert
• [ ] #152
• [x] #151
• [x] #136
• [x] #185
• [x] #201

Generalizations:

• [ ] Predefined filters: allow configuration of filter labels + regex
• [ ] Label parser: allow configuration of labels that need to be parsed and turned into links